1. Field of the Invention
The products and methods of the present disclosure relate generally to strengthened glass sheet and more particularly to glass sheet exhibiting high resistance to flexural strain and impact damage.
2. Technical Background
Cover glass sheet for consumer electronics devices including cell phones, PDAs, desktop, laptop and netbook computers, and LCD and plasma televisions is presently in high demand. The performance attributes of such sheet that are highly valued by designers, producers and end-users of these devices include low thickness, high surface strength and scratch resistance, and high resistance to flexural strain and impact damage.
Universally accepted methods for evaluating the resistance of cover glass sheet to impact damage such as cracking or shattering include standardized flexural (bending) strain and ball drop (impact) testing. Ball drop testing is quite demanding and can reveal wide ranges of impact damage resistance in even a single commercial cover glass sheet product line if close control over glass quality is not exercised. Thus tightly distributed ball drop failure performance results are as important as resistance to breakage at high ball drop heights.
Although ball drop testing is widely used for both system and component level testing, glass attributes that impact ball drop performance are complex. For example, increasing failure rates at higher ball drop heights and/or reduced cover glass sheet thicknesses or sheet strengths are generally expected and observed, but the variability in failure rates at a single ball drop height for glass sheet of common manufacturing origin, nominal thickness, and equivalent bending strength remains a major concern.
Moreover, glass strengthening methods that are highly effective to increase sheet glass resistance to ball drop impact breakage can still produce sheet exhibiting widely varying strengths under flexural tests designed to evaluate the bending modulus of rupture strengths of the glasses. For example, bi-axial or ring-on-ring flexural strength tests that are conducted on nominally identical glass samples pre-processed to improve impact damage resistance can produce widely varying flexural strength results.